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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Unconventional magnetotransport and antiferromagnetic order in Eu 2 ⁢InTe 5

Here, we report on the structural characterization and physical properties of Eu 2 ⁢InTe 5 , a distorted square-net motif antiferromagnetic semiconductor. Single crystal x-ray diffraction measurements reveal a distortion of the Te square-net layers such that an orthorhombic supercell is necessary to accurately describe the structure. Magnetization, resistivity, and specific heat measurements confirm antiferromagnetic ordering at 𝑇 𝑁 = 7.3K. Anisotropy in magnetization data suggests the magnetic moments are oriented parallel to the 𝑐 axis. We observe an unconventional transverse magnetoresistance and Hall resistivity in the paramagnetic state (𝑇 ≤ 80K), indicating strong coupling between localized Eu 2+ magnetic moments and conduction electrons. Resistivity and Hall measurements indicate the material is a heavily doped semiconductor, with impurity states related to disorder. Our findings highlight a relatively unexplored Eu-based antiferromagnetic semiconductor with unconventional magnetotransport behavior.

Cook, Matthew S. [Oak Ridge National Laboratory (O↗
DOE OSTI2025

Impurity-induced moment freezing in NaFe x⁢ Ru 1–x O 2

We report the impact of magnetic impurity substitution on the quantum disordered magnetic ground state of NaRuO 2 . Local S = 5/2 moments are introduced into the frustrated triangular lattice of J eff = 1/2 Ru moments via Fe substitution in NaFe x⁢ Ru 1–x O 2 , and the evolution of the magnetic ground state is reported. Local spin freezing associated with conventional spin-glass behavior is observed upon Fe substitution, marking an impurity-induced freezing of the primarily dynamic magnetic ground state in NaRuO 2 . Furthermore, local Fe moments induce a Curie-Weiss magnetic behavior in the uniform magnetic susceptibility, and the local moment magnitude is best described by dynamic Ru moments polarized about impurity sites. Furthermore, our results establish an impurity-doping phenomenology consistent with inherently dynamic moments in NaRuO 2 that are pinned by local magnetic impurities, similar to “swiss cheese” models of impurity-substituted copper oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Understanding Heterogeneities in Quantum Materials

Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entanglement, and topological effects in the host system. Therefore, a critical need is to understand the roles of heterogeneities in order to endow materials with new quantum functions for energy and quantum information science applications. In this article, several representative examples are reviewed on the recent progress in connecting the heterogeneities to the quantum behaviors of real materials. Specifically, three intertwined topic areas are assessed: i) Reveal the structural, electronic, magnetic, vibrational, and optical degrees of freedom of heterogeneities. ii) Understand the effect of heterogeneities on the behaviors of quantum states in host material systems. iii) Control heterogeneities for new quantum functions. This progress is achieved by establishing the atomistic-level structure-property relationships associated with heterogeneities in quantum materials. Finally, the understanding of the interactions between electronic, magnetic, photonic, and vibrational states of heterogeneities enables the design of new quantum materials, including topological matter and quantum light emitters based on heterogenous 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Material Effects on Electron-Capture Decay in Cryogenic Sensors

Several current searches for physics beyond the standard model are based on measuring the electron-capture (EC) decay of radionuclides implanted into cryogenic high-resolution sensors. The sensitivity of these experiments has already reached the level where systematic effects related to atomic state energy changes from the host material are a limiting factor. One example is a neutrino mass study based on the nuclear EC decay of 7 Be to 7 Li inside cryogenic Ta-based sensors. Here, to understand the material effects at the required level, we use density-functional theory to model the electronic structure of lithium atoms in different atomic environments of the polycrystalline Ta absorber film. The calculations reveal that the Li 1s binding energies can vary by more than 2 eV due to insertion at different lattice sites, at grain boundaries, in disordered Ta, and in the vicinity of various impurities. However, the total range of Li 1s shifts does not exceed 4 eV, even for extreme amorphous disorder. Furthermore, when investigating the effects on the Li 2s levels, we find broadening of more than 5 eV due to hybridization with the Ta band structure. Material effects are shown to contribute significantly to peak broadening in Ta-based sensors that are used to search for physics beyond the standard model in the EC decay of 7 Be, but they do not explain the full extent of observed broadening. Understanding these in-medium effects will be required for current- and future-generation experiments that observe low-energy radiation from the EC decay of implanted isotopes to evaluate potential limitations on the measurement sensitivity.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Neural Network Solver for Small Quantum Clusters

Machine learning approaches have recently been applied to the study of various problems in physics. Most of these studies are focused on interpreting the data generated by conventional numerical methods or the data on an existing experimental database. An interesting question is whether it is possible to use a machine learning approach, in particular a neural network, for solving the many-body problem. In this paper, we present a neural network solver for the single impurity Anderson model, the paradigm of an interacting quantum problem in small clusters. We demonstrate that the neural-network-based solver provides quantitative accurate results for the spectral function as compared to the exact diagonalization method. This opens the possibility of utilizing the neural network approach as an impurity solver for other many-body numerical approaches, such as the dynamical mean field theory.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Mg vacancy and impurity-limited MgO single crystal thermal conductivity

Magnesium oxide (MgO) exhibits one of the highest thermal conductivities among oxides and is widely used as a dielectric material and substrate in semiconductor devices, in refractory applications, and as a promising filler in thermal interface materials for electronics. Its high thermal conductivity may be sensitive to impurity and defects, yet this influence is still uncertain. Here, in this study, the impact of the common impurities, i.e., Al, Ca, Ti, V, Fe, Si, B, Nb, Zr, Na, and K, as well as Mg and O vacancies on phonon scattering and thermal conductivity of MgO is studied using a fully first-principles T-matrix framework. It is found that B, Nb, and Zr impurities, along with Mg vacancies, lead to exceptionally strong reductions in thermal conductivity. By contrast, O vacancies and other impurities have modest to minimal impacts. Leveraging the T-matrix results, we reassess the perturbative, mass-only formalism whose use is pervasive in the literature and show that neglecting bond disorder does not necessarily lead to underestimation: for all transition-metal impurities studied, bond perturbations partially cancel mass disorder, causing the traditional perturbative model to overestimate scattering. We propose a simple modified perturbative expression that incorporates both mass and bond disorder and closely reproduces the T-matrix trends. Our predicted low-temperature trends by including phonon-impurity and phonon-boundary scattering match reasonably well with experiments. This work provides an in-depth study of impurity- and vacancy-limited thermal conductivity of MgO and suggests that reported “high-purity” MgO values have likely not yet reached the intrinsic upper limit, which may be substantially higher.

36 - MATERIALS SCIENCE↗
DOE OSTI2026

Detecting impurity-specific effects on structure and radiolytic hydrogen production in aluminum hydroxide

While radiolytic hydrogen (H 2 ) generation is an intrinsic property of aqueous and mineral radiolysis in nuclear waste systems, detection of the sub-ns events leading to H 2 generation is challenging. Interfacial processes involving key mineral phases in the sludge, e.g., gibbsite (α-Al(OH) 3 ), have been implicated, with impurities affecting the amount of H 2 generated. To understand why gibbsite synthesized from nitrate precursors produces less H 2 than gibbsite from chloride precursors, we paired 27 Al multiple quantum magic angle spinning (MQMAS) NMR spectroscopy to determine structural heterogeneity with transverse-field muon spin rotation (TF-μSR) to probe electron availability. MQMAS revealed greater structural disorder in the gibbsite synthesized with nitrate (NO 3 -gibbsite). Correspondingly, TF-μSR showed a larger diamagnetic fraction for NO 3 -gibbsite, indicating reduced persistence of μ + -electron bound states (muonium or other radicals) and thus fewer electrons available for reaction on the sub-ns timescale. This establishes a correlation between impurity-induced disorder and electron loss. The diamagnetic fraction serves as a signature for these sub-ns events, as it provides a key constraint for predictive models without currently resolving whether the electron is lost to direct chemical scavenging or trapping at lattice defects.

Graham, Trent R. [Pacific Northwest National Labor↗
NASA NTRS2000

Protein Crystal Growth Dynamics and Impurity Incorporation

The general concepts and theories of crystal growth are proven to work for biomolecular crystallization. This allowed us to extract basic parameters controlling growth kinetics - free surface energy, alpha, and kinetic coefficient, beta, for steps. Surface energy per molecular site in thermal units, alpha(omega)(sup 2/3)/kT approx. = 1, is close to the one for inorganic crystals in solution (omega is the specific molecular volume, T is the temperature). Entropic restrictions on incorporation of biomolecules into the lattice reduce the incorporation rate, beta, by a factor of 10(exp 2) - 10(exp 3) relative to inorganic crystals. A dehydration barrier of approx. 18kcal/mol may explain approx. 10(exp -6) times difference between frequencies of adding a molecule to the lattice and Brownian attempts to do so. The latter was obtained from AFM measurements of step and kink growth rates on orthorhombic lysozyme. Protein and many inorganic crystals typically do not belong to the Kossel type, thus requiring a theory to account for inequivalent molecular positions within its unit cell. Orthorhombic lysozyme will serve as an example of how to develop such a theory. Factors deteriorating crystal quality - stress and strain, mosaicity, molecular disorder - will be reviewed with emphasis on impurities. Dimers in ferritin and lysozyme and acetylated lysozyme, are microheterogeneous i.e. nearly isomorphic impurities that are shown to be preferentially trapped by tetragonal lysozyme and ferritin crystals, respectively. The distribution coefficient, K defined as a ratio of the (impurity/protein) ratios in crystal and in solution is a measure of trapping. For acetylated lysoyzme, K = 2.15 or, 3.42 for differently acetylated forms, is independent of both the impurity and the crystallizing protein concentration. The reason is that impurity flux to the surface is constant while the growth rate rises with supersaturation. About 3 times lower dimer concentration in space grown ferritin and lysozyme crystals might be examples explaining higher quality of the space grown protein crystal. Depletion of solution with respect to isomorphic impurities around a growing crystal may be K times deeper than with respect to the crystallizing protein.

Chernov, Alex A.↗
DOE OSTI2024

Imaging Valley Excitons in a 2D Semiconductor with Scanning Tunneling Microscope-Induced Luminescence

Valley excitons dominate the optoelectronic response of transition-metal dichalcogenides and are drastically affected by structural and environmental inhomogeneities localized in these materials. Critical to understanding and controlling these nanoscale excitonic changes is the ability to correlate the imaging of excitonic states with crystalline structures on the atomic scale. Here, in this study, we apply scanning tunneling microscope-induced luminescence microscopy to image valley excitons in a semiconducting transition-metal dichalcogenide monolayer decoupled by a 10 nanometer-thick hexagonal-boron-nitride flake incorporated in a lateral homojunction on an Au electrode surface. This design enables the observation of chiral excitonic emission arising from neutral and charged valley excitons of the monolayer semiconductor at ambipolar voltages with a quantum efficiency up to similar to ~10 -5 photon/electron. The measured light helicity demonstrates considerable circular polarization dependent on the sample voltage, reaching as much as 40%. The real-space luminescence imaging maps$-$at subnanometer resolution$-$of the valley excitons reveal striking spatial variations associated with localized inhomogeneities, including surface impurities and possibly nanoscale dielectric and/or potential disorders in the monolayer. Our study introduces a promising format for 2D materials to explore and tailor their optoelectronic processes at the atomic scale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Electron Tunneling and X-Ray Photoelectron Spectroscopy Studies of the Superconducting Properties of Nitrogen-Doped Niobium Resonator Cavities

Scanning tunneling microscopy and spectroscopy and X-ray photoelectron spectroscopy (XPS) have been used to investigate the differences between the surface electronic structures and chemical structure of typically prepared and N-doped Nb cutouts from superconducting radio frequency (SRF) cavities. The goal of this work is to get insights into the fundamental physics and materials mechanisms behind the striking decrease of the surface resistance with the radiofrequency magnetic field, which has been observed by many groups on N-doped Nb cavities. In particular, we address the effects of N-doping on the superconducting properties at the surface of SRF cavities. XPS measurements reveal a significantly more oxidized Nb 3d states on the N-doped Nb surfaces which is confirmed by tunneling spectroscopy measurements. Analysis of the tunneling spectra in the framework of a recent model (A. Gurevich and T. Kubo Phys. Rev. B 96, 184515 (2017)) shows that the N-doping greatly reduces local distribution of superconducting properties on the surface, causes a significant shrinkage of the metallic suboxide and changes the contact resistance between the metallic suboxide and the bulk niobium toward an optimum value, resulting in a lower surface resistance. Combination of these factors enables one to effectively tune the density of states at the surface and to reveal the decrease of the surface resistance with the radio frequency field which follows from the BCS theory. A slightly reduced average gap and a smaller coherence length, revealed by tunneling spectroscopy in the vortex cores, have been found in the N-doped Nb samples compared to typically prepared Nb samples, indicating a stronger impurity scattering caused by nitrogen doping in a moderately disordered material.

Lechner, Eric M.↗
DOE OSTI2026

Chirality reversal at finite magnetic impurity strength and local signatures of a topological phase transition

Here, we study the honeycomb lattice with a single magnetic impurity modeled by adding imaginary next-nearest-neighbor hopping 𝑖⁢ℎ on a single hexagon. This Haldane defect gives a topological mass term to the gapless Dirac cones and generates chirality. For a small density of defects, Neehus et al. [Phys. Rev. Lett. 135, 126604 (2025)] found that the system's chirality reverses at a critical ℎ 𝑐 ≈ 0.95 associated with an unexpected tricritical point of Dirac fermions at zero defect density. We investigate this zero-density limit by analyzing a single defect and computing two experimentally relevant measures of chirality: (1) orbital magnetization via local Chern marker, a bulk probe of all occupied states; and (2) electronic currents of low-energy states. Both probes show a chirality reversal at a critical ℎ 𝑐 ≈ 0.9–1.0. Motivated by this consistency, we propose a defect-scale toy model whose low-energy states reverse their chirality at ℎ$^{'}_{c}$ ≈ 0.87. Remarkably, the same pair of zero-energy bound states also generates the critical point ℎ 𝑐 in the full impurity projected T-matrix. Our results show how the chirality reversal produced by an impurity can be observed either in local probes or in the global topology, and suggest a possible role of the microscopic defect structure at the critical point.

Chern insulators↗
DOE OSTI2022

Statics and dynamics of skyrmions interacting with disorder and nanostructures

Magnetic skyrmions are topologically stable nanoscale particlelike objects that were discovered in 2009. Since that time, intense research interest in the field has led to the identification of numerous compounds that support skyrmions over a range of conditions spanning from cryogenic to room temperatures. Skyrmions can be set into motion under various types of driving, and the combination of their size, stability, and dynamics makes them ideal candidates for numerous applications. At the same time, skyrmions represent a new class of system in which the energy scales of the skyrmion-skyrmion interactions, sample disorder, temperature, and drive can compete. A growing body of work indicates that the static and dynamic states of skyrmions can be influenced strongly by pinning or disorder in the sample; thus, an understanding of such effects is essential for the eventual use of skyrmions in applications. The current state of knowledge regarding individual skyrmions and skyrmion assemblies interacting with quenched disorder or pinning is reviewed. Additionally, the microscopic mechanisms for skyrmion pinning, including the repulsive and attractive interactions that can arise from impurities, grain boundaries, or nanostructures, are outlined. This is followed by descriptions of depinning phenomena, sliding states over disorder, the effect of pinning on the skyrmion Hall angle, the competition between thermal and pinning effects, the control of skyrmion motion using ordered potential landscapes such as one- or two-dimensional periodic asymmetric substrates, the creation of skyrmion diodes, and skyrmion ratchet effects. Highlighted are the distinctions arising from internal modes and the strong gyrotropic or Magnus forces that cause the dynamical states of skyrmions to differ from those of other systems with pinning, such as vortices in type-II superconductors, charge density waves, or colloidal particles. Throughout this review future directions and open questions related to the pinning and dynamics in skyrmion systems are also discussed.

36 MATERIALS SCIENCE↗
NASA NTRS2000

The Structure of Ice Nanoclusters and Thin-films of Water Ice: Implications for Icy Grains in Cold Molecular Clouds

The cubic to hexagonal phase transformation in water ice (I(sub c) yields I(sub h)) is used to measure the extent to which surface structure and impurities control bulk properties. In pure crystalline (I(sub c)) water ice nanoclusters and in thin-films of impure water ice, I(sub c) yields I(sub h) occurs at lower temperatures than in thin-films of pure water ice. The disordered surface of the 20 nm diameter nanoclusters promotes transformations or reactions which would otherwise be kinetically hindered. Likewise, impurities such as methanol introduce defects into the ice network, thereby allowing sluggish structural transitions to proceed. Such surface-related phenomena play an important role in promoting chemical reactions on interstellar ice grains within cold molecular clouds, where the first organic compounds are formed.

Delzeit, Lance↗
DOE OSTI2023

Disorder, interactions, and their interplay in novel narrow-gap Dirac materials and Weyl semimetals

Progress of the modern day condensed matter physics is to a large extent driven by the synthesis of new materials, advances in their experimental characterization and theoretical description. Recent discoveries of novel gapless Weyl semimetals, such as NaBi,CdAs, and BiTe-based films, in which magnetic dopants essentially suppress the gap, have added to the family of graphene and topological insulators actively investigated over the past decade. With the field of novel semimetals rapidly maturing, its focus necessarily shifts from demonstrations of the feasibility of such materials to their quantitative characterization. While the transport and optical properties of graphene and topological insulators are well captured within the picture of free non-interacting electrons, gapless 3D Weyl semimetals and narrow-gap 2D semiconductors with Dirac spectrum are known to be extremely susceptible to disorder and electron-electron interactions. This susceptibility obscures the manifestations of nontrivial band structure -- like quantum anomalous Hall effect -- of the new topological materials. Among particular projects to be addressed are: 1) optical conductivity of 3D gapless Dirac fermions in the presence of smooth disorder, 2) interplay of disorder and Coulomb interactions in the spectral properties of such fermions, 3) formation and structure of the impurity band with Coulomb supercritical clusters, 4) Coulomb interaction-driven renormalization of the electron spectrum and of the transport response in the presence of strong magnetic field, 5) instanton approach to the disorder-induced fluctuation states in zero-gap 3D materials, and 6) the role of disorder in quantum anomalous Hall effect. The proposal relies upon the investigators' previous broad expertise in interacting and disordered electron systems. The methods to be employed include perturbative diagrammatic technique, non-perturbative instanton and self-consistent approximations, hydrodynamics of electron liquid. Both analytical as well as numerical approaches are to be employed. The anticipated broader outcome of the proposal includes gaining an in-depth understanding of the interplay of the disorder and interactions under the conditions when this interplay has the most dramatic impact on observables. Traditionally, interaction effects are among the most challenging and interesting problems of condensed matter physics. Similarly, disordered systems typically present very difficult but extremely rich problems in the description of various materials. Importantly, understanding the spectral and transport properties of such materials not only presents the fundamental objective, but is also of particular interest for many applications, such as computation, memory, optics, plasmonics. In particular realization of the quantum anomalous Hall effect may lead to the development of low-power-consumption electronics. Indeed, a major constraint for practical use of the quantum Hall effect is limited by the requirement of the quantizing magnetic field. At the same time, the quantum anomalous Hall effect samples exhibit non-dissipative edge quantum transport in a zero magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Low-temperature T -linear resistivity due to umklapp scattering from a critical mode

In this study, we consider the transport properties of a model of fermions scattered by a critical bosonic mode. The mode is overdamped and scattering is mainly in the forward direction. Such a mode appears at the quantum critical point for an electronic nematic phase transition and in gauge theories for a U(1) spin liquid. It leads to a short fermion lifetime, violating Landau's criterion for a Fermi liquid. In spite of this, transport can be described by a Boltzmann equation. We include momentum relaxation by umklapp scattering, supplemented by weak impurity scattering. We find that above a very low temperature which scales with Δ q 3 , where Δ q is the minimum umklapp scattering vector, the resistivity is linear in T with a coefficient which is independent of the amount of disorder. This picture holds until the temperature dependent part of the resistivity exceeds that due to impurity scattering. We compare the relaxation time approximation with an exact numerical solution of the Boltzmann equation. Surprisingly we find that, unlike the resistivity, the Hall coefficient strongly deviates from the relaxation time approximation and shows a strong reduction with increasing temperature. We comment on possible comparisons with experiments on high T c cuprates.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Probing many-body dynamics in a two-dimensional dipolar spin ensemble

Abstract The most direct approach for characterizing the quantum dynamics of a strongly interacting system is to measure the time evolution of its full many-body state. Despite the conceptual simplicity of this approach, it quickly becomes intractable as the system size grows. An alternate approach is to think of the many-body dynamics as generating noise, which can be measured by the decoherence of a probe qubit. Here we investigate what the decoherence dynamics of such a probe tells us about the many-body system. In particular, we utilize optically addressable probe spins to experimentally characterize both static and dynamical properties of strongly interacting magnetic dipoles. Our experimental platform consists of two types of spin defects in nitrogen delta-doped diamond: nitrogen-vacancy colour centres, which we use as probe spins, and a many-body ensemble of substitutional nitrogen impurities. We demonstrate that the many-body system’s dimensionality, dynamics and disorder are naturally encoded in the probe spins’ decoherence profile. Furthermore, we obtain direct control over the spectral properties of the many-body system, with potential applications in quantum sensing and simulation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Mixed-strategy approach to band-edge analysis and modeling in semiconductors

In semiconductor physics, the Urbach rule describes an exponential dependence of the UV-Vis absorption coefficient on photon energy at the band edge. The rule is so ubiquitously applied that a single common mechanism, variability in the local potential, is generally considered to be the cause of exponential band edges. Variability in the local potential may be caused by alloy inhomogeneity, structural defects, impurities, thermal excitation, or any other mechanism that interrupts ideal crystallinity and creates disorder. A crucial point of the Urbach rule is its development for band edges which approach linearity when plotted on a logarithmic scale. The rule was not developed in the context of nonlinear band edges with complex fine structure. Even so, this model has been applied extensively to band edges with such features. Using differential analysis and deconvolution, we develop an analytical technique capable of separating the components responsible for the fine structure observed. This approach reduces challenging signal-artifact convolution problems to a simple linear combination. Furthermore, we develop a model that can be applied to the nonlinear band-edge signals extracted. In this work, we show that in the limiting case of linearity, our model is consistent with and produces the same results as the linear Urbach model. Our model is capable of bypassing “tailing” as an indirect measure of the spread in the local potential and gives a direct measure of this quantity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗